This invention relates generally to a power controller for industrial applications and, more particularly, to a silicon controlled rectifier (SCR) power controller.
Power controllers arc used in industrial applications to regulate power supplied to machinery, manufacturing equipment, and support systems such as heating and air conditioning systems, for example. In general, a power controller provides the interface between an electrical utility (e.g., user owned power generating facility or other supplier) and the electrical distribution network being served by the power supplied by the electrical utility. The power controller typically functions to supply power to using equipment under normal operating conditions, as well as to interrupt the supply of power in the event of overloads or other extraordinary circumstances, to prevent damage to the network and the equipment connected to the network.
In many industrial applications, power controllers require large, expensive installations involving numerous switches, sensors, and indicators for automatically distributing power through the electrical distribution network. Furthermore, if there is a possible malfunction with the power controller, it is important to be able to shut down the power controller in a controlled manner to prevent damage to the using equipment.
Most large electrical resistance heaters utilize a three-phase circuit in which each circuit is individually fused. For a particular installation, a power controller may handle three-phase voltages of between 208Vac to 600Vac, with currents ranging from 50A to 2500A. In such applications, a silicon controlled rectifier (SCR) type power controller may be used to regulate the amount of electricity supplied to the resistive load of a heater. For example, the power controller may employ SCRs gated on to allow current flow in a particular direction. Once gated into conduction, each SCR will continue conducting until current flow in the desired direction stops. For alternating current (AC) power distribution networks, each SCR is gated into conduction for each half-cycle of the AC input wave form during which current flows to a particular phase.
The output of a SCR type power controller is often connected to a power distribution unit and to a number of three-phase fuse blocks. For example, the output may be wired to six or eight of such fuse blocks. In addition, the power controller also may include a firing unit or firing package having outputs connected to respective gate inputs of each SCR. In such cases, control or gate inputs for the SCRs are supplied as inputs to the circuitry within the firing package, which in turn produces output signals for gating the SCRs into conduction at the proper times.
Every component of a power controller consumes valuable panel space and necessitates additional wiring to make electrical connections between components. Because each electrical connection represents a potential problem and increases the cost of a unit by requiring additional labor and materials, minimizing the number of connections is a continuous goal.
To date, power controllers have not been designed so as to minimize connections and reduce initial cost. For example, existing power controllers provide very limited access to components and only few options for running the power wiring. In particular, such power controllers typically include only a single opening in a side of the unit for running wiring in and out of the same end or at most two openings in opposed sides for running wiring in one end and out of the other end. Consequently, it is often necessary to bend power wiring within and/or around the power controller to accommodate various designs. In large power controllers, several square feet of space may be required for bending the power wiring so that proper electrical connections can be made. The space and bending radius of power wiring can easily exceed the footprint of the power controller, resulting in a much larger unit than would otherwise be necessary.
Moreover, in power controllers that are closed or xe2x80x9ctouch safe,xe2x80x9d wiring becomes even more complicated. Power controllers that are xe2x80x9ctouch safexe2x80x9d generally include a door, hinged top, or other cover for shielding components that become hot to the touch during use. Such protective measures further limit access to components and the ways in which the power wiring can be routed.
Accordingly, there exists the need for a compact, versatile, and xe2x80x9ctouch safexe2x80x9d power controller having a cost-effective design that accommodates multiple electrical connections for various power applications.
Furthermore, SCR power controllers are not perfect conductors and exhibit some voltage drop across the SCRs. When current is flowing through the SCRs, the voltage drop generates heat. Heat must be removed from the SCRs so that a safe operating temperature is not exceeded causing failure of the controller. Some manufacturers have provided a thermostat mounted on a heat sink to shut down the SCR power controller if temperature approaches the point where the SCR may be damaged. There are two problems with this approach. The first is that thermostats have proved very inaccurate, and the other is that shutting down production with no warning can prove very costly in many industrial processes.
Accordingly, there exists the need for a power controller having an improved mechanical design and self temperature monitoring capabilities.
In one general aspect, a power controller includes a first switching device and a first bus bar pair (an input bus bar and an output bus bar) mounted to the first switching device such that wiring may enter and exit the power controller. The first bus bar pair may have a face plate extending at least partially above opposed side plates of the power controller and a mounting plate in electrical contact with the first switching device.
Implementations may include one or more of the following features. For example, the wiring may enter and exit either end of the power controller in a direction substantially parallel to the opposed side plates. The wiring may be power wiring and/or load wiring.
The power controller may be configured to provide power distribution directly from the first output bus bar. The first output bus bar may be constructed of a plated copper material, such as tin-plated copper. The face plate of the first output bus bar may include a plurality of holes for accommodating connectors of different sizes, ranging from #8 AWG to 500 MCM. One or more lugs such as, for example, NEMA standard two hole copper crimp lugs, may be mounted to the first bus bar. The first bus bar may be electrically connected to one or more load circuits.
The power controller may be configured to supply at least one of single-phase power or three-phase power to using equipment. The power controller may include a firing package for controlling the first switching device. The firing package may include one or more plug and play cards, such as for example, a plug and play card for providing proportional control and/or providing shorted SCR detection. The firing package may include a jumper module for selecting between single-cycle control and three-cycle control. The firing package also may include a trigger board mounted on the switching device and containing the line voltage for triggering the first switching device. The trigger board may be configured for use with different sizes of switching devices by way of break-off tabs.
The power controller may be structured and arranged in a single-phase configuration, a three-phase two-leg configuration, and/or a three-phase three-leg configuration. The power controller may include a second bus bar, a third bus bar, and/or a fourth bus bar. The bus bars may be mounted such that wiring may enter and exit the power controller from either direction. The bus bars may each have a face plate extending at least partially above the opposed side plates of the power controller. The power controller may be configured to provide power distribution directly from the first bus bar, the second bus bar or the third bus bar. An insulator held in place by the bus bars and the switching device may be provided between bus bars.
In another general aspect, a power controller assembly includes a power controller having one or more bus bars and a removable cover having a top surface, opposed end surfaces, and opposed side surfaces for attaching to and enclosing the power controller. The opposed end surfaces may have one or more windows through which wiring may pass and connect to one or more of the bus bars of the power controller.
Implementations may include one or more of the following features. For example, the windows of the cover may be sealed with an insulator, such as a gasket material (e.g., styrene butadiene rubber). The gasket material may include slits through which wiring (e.g. power wiring, load wiring) may pass. The slits that are not used to pass wiring may be held together by tape. The insulator also may include an adhesive for adhering to the end surface of the cover.
Other features and advantages will be apparent from the following description, including the drawings, and from the claims.